1. Field of the Invention
The present invention relates to a document processing apparatus such as an electronic typewriter capable of inputting characters and printing the inputted characters.
2. Related Background Art
Two types of conventional typewriters are available which differ in their printing systems.
The first conventional typewriter employs a printing system for selecting a type corresponding to an input key from types arranged around the outer periphery of a daisy wheel and for performing printing using the selected type. The second conventional typewriter employs a printing system in which dot patterns constituting each character are stored in a nonvolatile memory, a dot pattern corresponding to an input character key is read out, and the input character is printed at a dot-matrix printing unit such as a thermal head.
The former typewriter has the advantage of performing high-quality printing of individual characters but has the disadvantage that the number of kinds of characters that can be printed is limited.
The latter typewriter has the advantage that it can print various kinds of characters (e.g., Ming types, Gothic types, and double-height characters) but has the disadvantage of poor printing quality. In recent years, along with improvements in printing heads, the number of dots constituting each dot pattern representing a character has increased. That is, printing quality of a character printed by this system can compete with that printed by the type of a daisy wheel printer.
In a conventional electronic typewriter employing a system for printing characters with dot patterns in units of lines, when a character (e.g., a double-height character) which extends over a plurality of lines is input, the printing head must print each character while vertically moving a sheet. Therefore, the printing time of each character is prolonged. In addition, when a plurality of characters extending over adjacent lines are to be printed on a single line, the sheet must be repeatedly vertically moved, thus causing a positional error during printing.
In order to eliminate the positional error, printing can be stopped until character information of one line is completely input, and printing in units of characters may be avoided. However, it then becomes impossible to check printing during typing, which simultaneity is an essential characteristic of typewriters.